Editorial: For the most part I hate describing audio system performance with numbers. Graphs contain far more extensive and detailed information providing a better picture. Many people, not technically knowledgeable, would like a simplistic "goodness meter" and latch onto numbers as a way to compare two audio devices. This is misguided at best; often misleading, propagating the blind leading the blind. See the "obsolete" 20 bit converter discussion concerning Stereophile, DAVE and Yggdrasil on several forums for a very good example of how easy it is to fall into the trap. Hint: there are NO DACs providing analog output resolving to an equivalent of 32 bits digital representation as an example.

Some observations on the numbers acquired:

1. 5243b THD+N is just wrong - not ruling out operator error yet

2. dScope THD+N at the top of the respective screenshots is a continuous measurement as opposed to the THD readings below which are averaged over 12 readings. It is possible for a given moment to indicate a LOWER THD+N than the THD. Over time the higher number is the one used. Very hard to synchronize a screen capture to a particular reading so there will be a few discrepancies between graphs and the chart of numbers.

3. The QA401 has an unusual signal generator output with signal amplitude ramping up and down. Measurement by other instruments expecting a continuous amplitude for asynchronous measurements are challenged to obtain a useful reading. Thus the dScope may indicate a number higher than that obtained by the QA401 which is synchronized to its own generator.

Editorial Conclusion (warning - personal opinion ahead):

The dScope offers a much greater set of measurements, pre-made measurement scripts, higher resolution, speed of measurements, etc. as well it should for the price. But given how well the QA401 performs within the constraints of what it was designed to measure, it represents an incredible value. If I were designing audio amps to retail, on a constrained budget, I would opt to own a QA401 and rent time as necessary for the larger set of measurements utilizing an AP, dScope, SRS, Rohde & Schwarz, or equivalent. I believe the QA401 should be part of any serious audio lab bench.

All devices had been powered on for several hours before measurements. However I noted that the HP339A THD was lower after 4 additional hours.

HP339A signal generator as measured by dScope after several hours

HP339A signal generator as measured by dScope with an additional 4 hours warmup

These lower numbers were not recorded in the main comparison chart as they were acquired inconsistent with the other devices. They are presented as an interesting anecdote supporting warmup over long periods of time benefiting audio devices.

Last thought - those engineers at HP were what I considered Real Engineers(TM). Seeing a 35+ year old device still performing to spec is enlightening. The 339 input level RMS readings were compared to a recently calibrated Fluke 189 DMM as an example. Dead on for every signal source Vrms measured.

The dScope offers a much greater set of measurements, pre-made measurement scripts, higher resolution, speed of measurements, etc. as well it should for the price. But given how well the QA401 performs within the constraints of what it was designed to measure, it represents an incredible value. If I were designing audio amps to retail, on a constrained budget, I would opt to own a QA401 and rent time as necessary for the larger set of measurements utilizing an AP, dScope, SRS, Rohde & Schwarz, or equivalent. I believe the QA401 should be part of any serious audio lab bench.

Click to expand...

QA401 is offering ASIO "unofficial" support. That opens the door to ARTA and REW. Furthermore, there is support for C#.

I'm getting really tempted to get this now. Check the ARTA test they ran:

EDIT: They said the ASIO was a bit tricky in their hypermiling blog post. However the recent release (like 4 days ago) is very encouraging:

"Added ASIO driver. Only left channel 48K atten active is functional right now. See "readme" in driver directory for instructions on how to install as its not installed by default. This driver won't be actively supported by us,, but we'd love to hear from you if it works as provided. With this driver, you should be able to move back and forth between Arta and the QA401 software. Note, though, that both apps cannot be running at the same time. if you have an Asio app open and try to open the QA401 app (or vice versa), the USB open will fail."

@atomicbob, I think you got the same results they got (or very close). The hypermiling discussion is all about the different THD and THD+N loopback numbers the unit gets depending on the input and output ports they use.

In their test, the worst set of numbers happen when they loopback R+ to R+ (-112.4 dB THD). Second worse is L- to L+ (-113.8 dB THD). However, they get the best numbers with R- to R+ (-119.8 dB THD). They encourage to select the best loopback ports to get the best results when doing a measurement.

Your THD+N is roughly the same (loopback is dominated by noise in the given BW): -90.5 or so.

They also argue that using a Hanning window degrades performance by like 1 dB or so, and they used Rectangular. With windows AFAIK is always a trade off with side lobes and main lobe width. They get what they get I guess.

So I think the unit you got is about right. It maybe possible to get an even better measurement channel by using a different combination of I/O.

If you want to give it a go, try switching to Rectangular window and check the loopback that yields the best results. I think they that the best I/O combo is different from unit to unit.

Still, the point being this is pretty impressive for an inexpensive analyzer as was mentioned in the first post. One of the reasons it is often used in my lab for tasks not requiring additional capabilities of the dScope.